The winners of the SPS Awards 2012

Nanoelectromechanical resonators are of great interest due to their potential applications for on-chip radio-frequency signal processing, sensing experiments with unprecedented sensitivity (mass spin, force, charge), and tests of quantum mechanics in extended bodies. Carbon-based resonators, in particular, appear very promising owing to their low mass, high structural quality, and high Young modulus.

Damping is central to the physics of nanoelectromechanical resonators; for instance, it lies at the core of quantum and sensing experiments. Damping has been successfully described by a linear damping force for all the mechanical resonators studied so far in vacuum, whose dimensions span many orders of magnitudes down to a few tens of nanometres.

In our work [1], we report that the linear damping scenario breaks down for resonators made from carbon nanotubes and graphene sheets, whose transverse dimensions are on the atomic scale. Indeed, we find that the damping in these resonators is much better explained by a nonlinear damping force.

Our finding has profound consequences. It entails that many predictions for NEMS resonators, e.g. concerning tests of quantum mechanics in extended bodies, ought to be revised when applied to nanotube/graphene resonators (since they were calculated assuming linear damping). In addition, our work provides a simple method to increase the mechanical quality factor in nanotube/graphene resonators, which is a matter of importance for many applications.

_{[1] A. Eichler, J. Moser, J. Chaste, M. Zdrojek, I. Wilson-Rae, and A. Bachtold, Nature Nanotech. 6, 339 (2011)}

We report transport experiments through graphene quantum dots and narrow graphene constrictions. In a quantum dot, electrons are confined in all dimensions, offering the possibility for detailed investigation and controlled manipulation of individual quantum systems. The recently isolated two-dimensional graphene is an interesting new material to study quantum phenomena. Due to its novel electronic properties and the expected weak interaction of the electron spin with the atomic nuclei, graphene quantum dots have been proposed as promising hosts for spin based quantum bits.

As graphene is a zero gap semiconductor, tunable carrier confinement poses a challenge. We fabricate graphene quantum dots by etching mono-layer flakes into 60-350 nm sized islands with narrow constrictions to the leads. Transport through the constrictions is suppressed around the electron-hole crossover and they can be used as tunable tunneling barriers for transport spectroscopy of the dot. Electron confinement in graphene quantum dots is observed by measuring Coulomb blockade and transport through excited states, a manifestation of quantum confinement [1].

In order to understand the spectrum of a quantum dot it is usually necessary to reach a regime with only one or two electrons in the dot. Measurements in a magnetic field perpendicular to the sample plane allow identifying the electron-hole transition regime via the crossover to the graphene specific zero-energy Landau level at high fields. After rotation of the sample into parallel magnetic field orientation, Zeeman spin-splitting with a g-factor of approximately two is measured. A g-factor of two is expected in the absence of spin-orbit interaction. The filling sequence of subsequent spin states showed no signatures of shell filling so far. This is attributed to the non negligible influence of exchange interactions among the electrons and low energy edge states [2]. These studies open the way for a more advanced understanding and control of spin states in graphene quantum dots.

_{[1] J. Güttinger, "Graphene Quantum dots" PhD thesis, ETH Zürich (2011)
[2] J. Güttinger, T. Frey, C. Stampfer, T. Ihn, and K. Ensslin, "Spin States in Graphene Quantum Dots" PRL 105, 116801 (2010)}

Scanning probe techniques, such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM), have been used to study both the electronic and the structural properties of molecules on surfaces with submolecular resolution [1,2]. However, the charge distribution within a molecule is not directly accessible with these techniques. Kelvin probe force microscopy (KPFM, a special mode of AFM), on the other hand, can be used to measure the local contact potential difference between the scanning probe tip and the surface, a quantity closely related to the charge distribution on the surface, but no submolecular resolution had so far been reported. The goal of our work was to combine the charge sensitivity of KPFM with the high resolution of STM and AFM, and thereby image the charge distribution within a molecule.
We investigated the molecule naphthalocyanine on an ultrathin insulating NaCl film on Cu(111) with a combined low-temperature STM and AFM setup. Naphthalocyanine consists of four lobes that give it a cross-like shape, and it has two hydrogen atoms at its center that can be switched between two different configurations in a process called tautomerization switching. Using STM and AFM in combination with density functional theory calculations, we found that the lobes parallel to the inner hydrogen atoms exhibit a lower electron density than the other two lobes. We were then able to show that this charge asymmetry clearly manifests itself in an asymmetric appearance of submolecularly resolved KPFM images. Also, the possibility of switching the tautomerization state of the molecule helped us exclude that the contrast is affected, for example, by an asymmetric shape of the scanning probe tip. We were able to identify the electric field generated by the inhomogeneous charge distribution within the molecule as the source of contrast in our KPFM images. Furthermore, we found that functionalizing the scanning probe tip with a single carbon monoxide molecule greatly enhanced the KPFM resolution, enabling for the first time the imaging of atomic-scale variations of the local contact potential difference over a molecule.
Our findings open up the possibility of directly imaging the charge distribution within charge-transfer complexes, which hold promise for future applications such as solar photoconversion or energy storage. Furthermore, it will now be possible to investigate how charge is redistributed when individual chemical bonds are formed between atoms and molecules on surfaces.

_{[1] J. Repp, G. Meyer, S. M. Stojkovic, A. Gourdon, and C. Joachim, Phys. Rev. Lett. 94, 026803 (2005).
[2] L. Gross, F. Mohn, N. Moll, P. Liljeroth, and G. Meyer, Science 325, 1110 (2009).}

Photovoltaics, the direct conversion of light into electricity, is a promising source of renewable energy to provide low cost solar electricity in a safe and sustainable manner. Among various thin film technologies, solar cells based on Cu(In,Ga)Se₂ absorber layers have yielded highest conversion efficiencies. While most of the R&D is carried out on rigid glass substrates, the use of flexible polymer foils as a substrate offers several advantages for lowering manufacturing and installation costs, as well as opening up new applications of lightweight flexible solar modules. However, as the conversion efficiency is an absolutely important factor for cost-competitiveness, it is necessary to obtain similar efficiencies on flexible substrates as on rigid ones, which up to now has not been achieved. The main reason has been that "high-quality" absorber layers were grown at rather high substrate temperatures of about 600°C, while polymer foils restrict the applicable growth temperature to well below 500°C. Lower substrate temperatures during absorber deposition have generally resulted in significantly lower efficiencies on polymer foils, and it was previously unknown which critical issues restrict the performance of such solar cells.

In this work, the composition gradient along the absorber layer, which is directly connected to the electronic band gap structure of the solar cell, has been identified as the main reason for inferior performance of such solar cells grown on plastic substrates at low temperature. For that purpose a specially designed multi-stage growth process has been invented which enables tuning of the composition grading and thereby engineering the band gap structure of the solar cell. Solar cells grown by this method exhibited similar conversion efficiencies as compared to devices that typically could only be obtained on rigid glass substrates and much higher growth temperatures. Adjustment of the compositional grading in an appropriate manner was found to be key to remove an electronic barrier for the minority carriers which otherwise results in enhanced recombination and reduced device performance. A best conversion efficiency of 18.7% was achieved on polyimide substrate which is a world-record for any type of solar cell grown on a flexible substrate. This achievement could have a broad impact on future PV manufacturing and holds great potential for a paradigm shift in the use of effective solar electricity production.

_{[1] A. Chirilă, S. Buecheler, F. Pianezzi, P. Bloesch, C. Gretener, A. R. Uhl, C. Fella, L. Kranz, J. Perrenoud, S. Seyrling, R. Verma, S. Nishiwaki, Y. E. Romanyuk, G. Bilger, and A. N. Tiwari, Highly efficient Cu(In,Ga)Se2 solar cells grown on flexible polymer films, Nature Materials 10, 857–861, 2011.
[2] A. Chirilă, P. Bloesch, S. Seyrling, A. Uhl, S. Buecheler, F. Pianezzi, C. Fella, J. Perrenoud, L. Kranz, R. Verma, D. Guettler, S. Nishiwaki, Y. E. Romanyuk, G. Bilger, D. Brémaud, and A. N. Tiwari, Cu(In,Ga)Se2 Solar Cell Grown on Flexible Polymer Substrate with Efficiency exceeding 17%, Prog. Photovolt.19, 560-564, 2010.}